Two projector projection and stereoscopic projection for large screens is increasing in use. In some cases, the stereoscopic effect can be achieved with a single projector, capable of delivering a left eye image and a right eye image to the observer's left and right eyes respectively, instead of the traditional approach of using two projectors. The market demand for single projector solutions is caused by the desire to eliminate difficult alignment of the two projectors. However, for larger screens, demands in terms of resolution and brightness may go as high as 4K resolution and near a 100.000 lumen. Large screens for two-dimensional (2D) presentations may require two projectors to obtain an acceptable level of image illumination. Single projector technology may not scale easily up to such requirements and larger screens accounts to less than 1% of the hundreds of thousands of projection screens worldwide, so the market may not be able to drive the large investments in development needed for such a single projector solution in the foreseeable future.
Hence, big screens demanding 4K resolution and up towards 100,000 lumen brightness may still need dual projector solutions. In large screen projection, alignment of the two projectors can be difficult, but important. The traditional way of mechanically adjusting the projectors may not be satisfactory for large screens because of limited tolerances in mechanical positioning systems and tolerances in optics, which can make it impossible for a perfect alignment of the projectors. A warping system can be included in one of the projectors, so a technician can electronically adjust a geometric correction, for example by the use of a reference grid, until the images are aligned. This manual alignment is cumbersome and time consuming work. Some systems incorporate auto-alignment systems, where the system can perform automatic alignment, running through an iterative process with a feedback camera.
Two projector systems can experience spatial changes in image alignment between the two projected images as a result of thermal changes that may occur and cause image shifts in a relatively short amount of time such as within a duration of a presentation. For example, a laser projection system may experience a relatively quick change in optic performance and cause a global horizontal, vertical, or rotational shift of the image. Therefore, having a means to realign the displayed images automatically during a presentation without being visible to a viewer would be beneficial.
FIG. 1 shows a configuration of prior art. The solid, arrowed lines are image signal paths. A projector pair that includes a first projector 1 and a second projector 2 is projecting an image pair such as a left eye image delivered by a first image generator 4 and a right eye image delivered by a second image generator 5 onto a projection surface 3. A warping system 6 is capable of geometrically correcting the left eye image, so it is projected to align with the right eye image. The warping system 6 may be capable of doing the geometric correction in real time during, for example, a presentation of a moving image. A camera 7 can record the images projected onto the projection surface 3 and deliver the recorded image data to a calibration system 8 that is capable of processing the received camera data, delivering geometry correction data to the warping system 6 and outputting calibration image sequences suited for alignment in an iterative closed-loop feedback system that will run through an alignment process over some seconds or minutes. A switch 9, can switch projectors between the image generators for content presentation and the output of the calibration circuit for auto-alignment. This configuration is an advantage over manual alignment, though for big screen installations operating over many hours at a time it may be necessary to perform realignment several times during the operation because of thermally and otherwise induced movements in mechanical and optical parts, and if the auditorium is filled with audience most of the time, it is not optimal to have to perform distracting realignments that are noticeable by the audience. If the realignment is to occur while the projection system is showing image content, then it is desirable that the alignment is done in a way that is invisible to viewers.
Systems and methods are desirable to overcome the above mentioned obstacles and provide for low perceptibility or invisible automatic alignment, that may in some circumstances display calibration images sequences for a fraction of a second, hence in practical use often go unnoticed, and further eliminate the need for projector switching and closed-loop feedback circuits.